1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gas. More particularly, it relates to a catalyst for purifying the exhaust gas from the internal combustion engine such as of an automobile for simultaneous removal from the exhaust gas of noxious components such as hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx), which catalyst especially exhibits outstanding durability even when it is used under harsh conditions such as under a high-temperature oxidative atmosphere and manifests a high purifying ability to the aforementioned noxious components at low temperatures.
2. Description of the Prior Art
In the conventional noble metal-containing catalyst for purifying the exhaust gas, for the purpose of ensuring effective use of the noble metal contained in a very minute amount in the catalyst, efforts have been made to allow the noble metal to be deposited in as high a degree of dispersion as possible on a refractory inorganic oxide of a large surface area such as activated alumina. The catalyst having the noble metal carried in a high degree of dispersion enjoys a high initial activity. When it is exposed to such harsh conditions as involved under a high-temperature oxidative atmosphere, however, the noble metal gains gradually in particle size, undergoes a chemical conversion into a less active state, and tends to induce a reaction with the carrier substance and cerium oxide. Because the noble metal is deposited in the high degree of dispersion, there tends to ensue a disadvantage that the degradation of catalytic activity is rather heavy.
In this field, zirconia is used more often than not as incorporated chiefly in a carrier substrate for the purpose of stabilizing the physical properties of the catalyst such as specific surface area. To cite a case using zirconia as a carrier substrate for a noble metal, Japanese Patent Publication No. SHO 57(1982)-29,215 and Japanese Patent Laid-Open No. SHO 57(1982)-153,737 disclose a method which comprises forming on a carrier a coating layer containing alumina and zirconia and subsequently depositing a noble metal thereon. The catalyst produced by the method of this principle, however, suffers from the degradation of catalytic activity due to the same cause as mentioned above because the greater part of the noble metal is substantially dispersed in a high ratio in the alumina.
As carrier substances incapable of interacting noble metals, particularly rhodium, zirconia (U.S. Pat. No. 4,233,189) and alpha alumina (U.S. Pat. No. 4,172,047) have been known in the art. Zirconia and alpha alumina generally possess small surface areas. It has been pointed out, however, that the catalysts having rhodium carried on these substances have a disadvantage that exhibit poor initial activity and possess no satisfactorily high ability to purify the exhaust gas at low temperatures after long term using.
An object of this invention, therefore, is to provide a novel catalyst for purifying the exhaust gas and a method for the production thereof.
Another object of this invention is to provide a catalyst for purifying the exhuast gas which exhibits outstanding durability even when it is used under harsh conditions and possesses a notable ability to purify thoroughly the exhaust gas of the noxious components thereof even at low temperatures and a method for the production thereof.